The chronic effects of environmental chemicals or of mixtures of chemicals are determined in suitable animal models. Factors influencing the outcome such as sex, strain, and species of animal are investigated. BIBLIOGRAPHIC REFERENCE: Ulland, B.M., Page, N.P., Squire, R.A., Weisburger, E.K., and Cypher, R.L.: A carcinogenicity assay of Mirex in Charles River CD rats. J. Natl. Cancer Inst. 58: 133-140, 1977.